Ionization of Thin and Thick Water Films on Platinum Field Emitter Tips - A Comparison of Theoretical and Empirical Trends
Abstract
Field ionization of thin water layers adsorbed onto a platinum field emitter tip was investigated by numerical simulation and analysis of experimental data. The numerical simulation, which includes a field-dependent relative permittivity, was developed to predict the field distribution around a water- covered field emitter tip. The simulation predicts that ionization begins at the water-vacuum interface in thin layers and at the tip-water interface in thick layers. Experiments were conducted to verify the predicted trends. Water was vapor deposited onto a cryogenically cooled tip under field-free conditions. Field ionization was probed by ramped field desorption (RFD) in which desorption of ionic species (hydrated protons) is measured while increasing the applied electric field linearly in time. The ionization onset field decreased from 0.5 to 0.2 V/A as temperature increased from 105 to 150 K. The estimated average relative permittivity for crystalline ice is 2.5 and that of amorphous ice is 2.0. The experimental results are consistent with the trends predicted by the numerical model. Data at all temperatures show a change in slope when the water thickness exceeds a critical thickness. This change in slope is evidence of a change in ionization location. The measured slope agrees with the predicted slope for thin crystalline layers, but not for amorphous layers. Thinning of the amorphous layers due to field-enhanced mobility of the ice may explain the discrepancy with the model.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 24, 1999
- Accession Number
- ADA365080
Entities
People
- Dawn L. Scovell
- Eric M. Stuve
- Tim D. Pinkerton
- Valentin K. Medvedev
Organizations
- University of Washington